The present invention relates to a transmission for automotive vehicles and the like, and more particularly relates to a transmission which is a combination of a main transmission device and a subtransmission device.
In a copending U.S. patent application Ser. No. 194,380 filed Oct. 6, 1982, now U.S. Pat. No. 4,368,650 made by the same applicants as, and assigned to the same assignees as, the present application, there has been proposed a transmission system for an automotive vehicle which incorporates a main transmission device which is a gear selection type transmission which may be manually shifted to a plurality--in most cases three or more--transmission stages, and which further incorporates an overdrive transmission device or a subtransmission device, which is a planetary gear type transmission which may be shifted between a high speed stage and a low speed stage, i.e., in most cases, a speed increasing stage and a direct connection stage. By the independent shifting of such a main transmission device and such a subtransmission device the number of gear ratios available for use by the driver of the automotive vehicle is increased, and accordingly the drivability of the vehicle, and its fuel consumption and polluting characteristics, are improved.
In such a transmission, as has been described in detail in connection with the aforementioned patent application, a ring gear incorporated in the subtransmission device is connected to the end of an intermediate shaft, which is incorporated in the main transmission device, and which carries several gear wheels for driving a power output shaft. In general in modern transmissions it is preferable to use helically cut gear wheels rather than plain or straight cut gear wheels, because helical gears are generally stronger than plain gears, produce less noise during transmission of rotational power, and last longer. These advantages are usually decisive in the design process. However, when a pair of helical gears are transmitting rotational power, each of them is subjected to an axial force, because of the fact that the teeth of the gears slope where they mesh, and are obliquely skew to their rotational axis rather than parallel thereto. The magnitude of this axial force due to the contact between a pair of teeth of the meshing gears is, substantially, proportional to the force in the circumferential direction of the gears between these teeth, multiplied by the tangent of the angle of skewing of the teeth, i.e. the angle between the directions of the teeth and the rotational axes of the gears. Further, the direction of the axial force on a helical gear, in terms of the rotational direction thereof, of course depends upon the handedness of the gear, i.e., depends on whether its teeth describe left handed or right handed spirals. In such a transmission as outlined above, using helically cut gears, there is therefore a problem of axial loading on the intermediate shaft, which may wear out the bearings upon which the intermediate shaft is supported in the transmission casing, especially if these are radial ball bearings. This problem is particularly troublesome when the subtransmission is being operated in its speed increasing stage, because in this case two separate axial forces are being exerted upon the intermediate shaft: an axial force generated by the meshing of the helical planetary pinion gear of the subtransmission with the helical ring gear thereof, which is transmitted from the ring gear to the intermediate shaft; and also an axial force which is exerted upon the helical gear within the main transmission which is being used for transmitting rotational power to the power output shaft. The combination of these two axial forces can very often prove so powerful as to significantly shorten the life of a bearing supporting the intermediate shaft.